Power apparatus applied in SST structure and three-phase power source system having the same

ABSTRACT

A power apparatus applied in an SST structure includes a first AC-to-DC conversion unit, a first DC bus, an isolated transformer, a DC-to-AC conversion unit, a second AC-to-DC conversion unit, and a second DC bus. The first AC-to-DC conversion unit has a first bridge arm and a second bridge arm. The first DC bus provides a first DC voltage. The isolated transformer has a primary side and a secondary side. The DC-to-AC conversion unit has a third bridge arm and a fourth bridge arm. The second AC-to-DC conversion unit has a fifth bridge arm and a sixth bridge arm. The second DC bus provides a second DC voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuing application of U.S. patent applicationSer. No. 16/562,074, filed on Sep. 5, 2019, and entitled “POWERAPPARATUS APPLIED IN SST STRUCTURE AND THREE-PHASE POWER SOURCE SYSTEMHAVING THE SAME”, which claims priority to Application No.201910624806.0 filed on Jul. 11, 2019 in China. The entire disclosuresof the above applications are all incorporated herein by reference.

BACKGROUND Technical Field

The present disclosure relates to a power apparatus and a three-phasepower source system having the same, and more particularly to a powerapparatus applied in SST structure and a three-phase power source systemhaving the same.

Description of Related Art

The statements in this section merely provide background informationrelated to the present disclosure and do not necessarily constituteprior art.

With the development of power electronic components and the developmentof decentralized power supplies and smart power grids, solid statetransformers (SST) have become an increasingly hot research topic. Solidstate transformers have multi-functional and high-performance features,including integration of microgrid, correction of power factor,compensation of reactive power, isolation of fault current, adjustmentof output voltage, and so on.

However, the power supply apparatus applied to the solid statetransformer structure still faces problems that need to be solved, suchas DC-side voltage balance, difficulty in wiring design, high labor andcost, complicated control circuit, incapable volume reduction, and soon. Therefore, how to design a power apparatus applied to in SSTstructure and a three-phase power source system having the same to solvethe aforementioned technical problems is an important subject studied bythe inventors of the present disclosure.

SUMMARY

An object of the present disclosure is to provide a power apparatusapplied in an SST structure to solve the mentioned-above problems.

In order to achieve the above-mentioned object, the power apparatusapplied in the SST structure includes a first AC-to-DC conversion unit,a first DC bus, an isolated transformer, a DC-to-AC conversion unit, asecond AC-to-DC conversion unit, and a second DC bus. The first AC-to-DCconversion unit has a first three-level bridge arm and a secondthree-level bridge arm coupled to the first three-level bridge arm, anda first side of the first AC-to-DC conversion unit is coupled to an ACpower source. The first DC bus is coupled to a second side of the firstAC-to-DC conversion unit, and has a first DC voltage. The isolatedtransformer has a primary side and a secondary side. The DC-to-ACconversion unit has a third three-level bridge arm and a fourththree-level bridge arm coupled to the third three-level bridge arm, anda first side of the DC-to-AC conversion unit is coupled to the first DCbus and a second side of the DC-to-AC conversion unit is coupled to theprimary side. The second AC-to-DC conversion unit has a fifththree-level bridge arm and a sixth three-level bridge arm coupled to thefifth three-level bridge arm, and a first side of the second AC-to-DCconversion unit is coupled to the secondary side. The second DC bus iscoupled to a second side of the second AC-to-DC conversion unit, and hasa second DC voltage.

Accordingly, the power apparatus applied in the SST structure isprovided to make the layout easy, simply the design of the controlcircuit, and reduce the occupied volume of circuits.

Another object of the present disclosure is to provide a three-phasepower system applied in an SST structure to solve the mentioned-aboveproblems.

In order to achieve the above-mentioned object, any one phase of an ACpower source is connected to a plurality of power apparatuses. The firstAC-to-DC conversion units of the power apparatuses are coupled inseries, and the second DC buses of the power apparatuses are coupled inparallel.

Accordingly, the three-phase power system applied in the SST structureis provided to make the layout easy, simply the design of the controlcircuit, reduce the occupied volume of circuits, and achieve the voltageequalization and power balance.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the present disclosure as claimed. Otheradvantages and features of the present disclosure will be apparent fromthe following description, drawings, and claims.

BRIEF DESCRIPTION OF DRAWINGS

The present disclosure can be more fully understood by reading thefollowing detailed description of the embodiment, with reference made tothe accompanying drawings as follows:

FIGS. 1A and 1B are circuit diagrams of a power apparatus applied in SSTstructure according to the present disclosure.

FIG. 2 is a block diagram of a three-phase power source system appliedin SST structure according to the present disclosure.

FIG. 3 is a schematic view of an interleaved phase-shift control of afirst AC-to-DC conversion unit according to the present disclosure.

DETAILED DESCRIPTION

Reference will now be made to the drawing figures to describe thepresent disclosure in detail. It will be understood that the drawingfigures and exemplified embodiments of present disclosure are notlimited to the details thereof.

Please refer to FIG. 1A and FIG. 1B, which show a circuit diagram of apower apparatus applied in SST structure according to the presentdisclosure. The power apparatus applied in SST structure includes afirst AC-to-DC conversion unit 11, a first DC bus 12, an isolatedtransformer 13, a DC-to-AC conversion unit 14, a second AC-to-DCconversion unit 15, and a second DC bus 16.

The first AC-to-DC conversion unit 11 has a first three-level bridge arm111 and a second three-level bridge arm 112 coupled to the firstthree-level bridge arm 111. A first side of the first AC-to-DCconversion unit 11 is coupled to an AC power source Vac. The first DCbus 12 is coupled to a second side of the first AC-to-DC conversion unit11, and has a first DC voltage Vdc1. The isolated transformer 13 has aprimary side 131 and a secondary side 132.

The first AC-to-DC conversion unit 11 is coupled to the AC power sourceVac (also referred to as “electric power grid”) through an EMItransformer and a boost inductor.

The DC-to-AC conversion unit 14 has a third three-level bridge arm 141and a fourth three-level bridge arm 142 coupled to the third three-levelbridge arm 141. A first side of the DC-to-AC conversion unit 14 iscoupled to the first DC bus 12 and a second side of the DC-to-ACconversion unit 14 is coupled to the primary side 131 of the isolatedtransformer 13. The DC-to-AC conversion unit 14 switches the voltage ofthe first DC bus 12 into a high-frequency AC signal and transmits thehigh-frequency AC signal through the isolated transformer 13. The secondAC-to-DC conversion unit 15 has a fifth three-level bridge arm 151 and asixth three-level bridge arm 152 coupled to the fifth three-level bridgearm 151. A first side of the second AC-to-DC conversion unit 15 iscoupled to the secondary side 132 of the isolated transformer 13. Thesecond DC bus 16 is coupled to a second side of the second AC-to-DCconversion unit 15, and has a second DC voltage Vdc2. The secondAC-to-DC conversion unit 15 receives the high-frequency AC signal of thesecondary side 132 of the isolated transformer 13, and converts thehigh-frequency AC signal into the second DC voltage Vdc2.

The first three-level bridge arm 111 has two in-series power switchesS11, S12 and a diode D1 is coupled to a common contact between the powerswitches S11, S12, and two in-series power switches S13, S14 and a diodeD2 is coupled to a common contact between the power switches S13, S14.The power switch S12 is coupled to the power switch S13 and commonlyconnected at a first contact P11, and the diode D1 is coupled to thediode D2 and commonly connected at a potential neutral point P10 so asto form a three-level bridge arm structure and to output three differentvoltage levels (shown in FIG. 3) by the three-level bridge armstructure. The power switches S11-S14 may be MOSFETs, IGBTs withanti-parallel diodes, or other semiconductor switches. Moreover, thepower switches S11-S14 and the diodes D1, D2 can be integrated into amodular structure, thereby reducing pins, making the layout easy, andreducing the variance of the components.

The second three-level bridge arm 112 has two in-series power switchesS15, S16 and a diode D3 is coupled to a common contact between the powerswitches S15, S16, and two in-series power switches S17, S18 and a diodeD4 is coupled to a common contact between the power switches S17, S18.The power switch S16 is coupled to the power switch S17 and commonlyconnected at a second contact P12, and the diode D3 is coupled tot thediode D4 and commonly connected at the potential neutral point P10. Thepower switches S15-S18 may be MOSFETs, IGBTs with anti-parallel diodes,or other semiconductor switches. Moreover, the power switches S15-S18and the diodes D3, D4 can be integrated into a modular structure. Inaddition, the first contact P11 and the second contact P12 of the firstAC-to-DC conversion unit 11 are coupled to the AC power source Vac.Since the first three-level bridge arm 111 and the second three-levelbridge arm 112 can respectively output three voltage level, a voltagebetween the first contact P11 and the second contact P12 can becontrolled in different voltage levels, such as Vdc1, ½*Vdc1, 0,−½*Vdc1, −Vdc1, thereby reducing switch stress and reducing harmonics.The manner of generating the control signal may be, for example but notlimited to, a space vector pulse width modulation (SVPWM). As long asdifferent voltage levels can be generated between the first contact P11and the second contact P12.

The third three-level bridge arm 141 has two in-series power switchesS21, S22 and a diode D5 is coupled to a common contact between the powerswitches S21, S22, and two in-series power switches S23, S24 and a diodeD6 is coupled to a common contact between the power switches S23, S24.The power switch S22 is coupled to the power switch S23 and commonlyconnected at a third contact P13, and the diode D5 is coupled to thediode D6 and commonly connected at the potential neutral point P10. Thepower switches S21-S24 may be MOSFETs, IGBTs with anti-parallel diodes,or other semiconductor switches. Moreover, the power switches S21-S24and the diodes D5, D6 can be integrated into a modular structure.

The fourth three-level bridge arm 142 has two in-series power switchesS25, S26 and a diode D7 is coupled to a common contact between the powerswitches S25, S26, and two in-series power switches S27, S28 and a diodeD8 is coupled to a common contact between the power switches S27, S28.The power switch S26 is coupled to the power switch S27 and commonlyconnected at a fourth contact P14, and the diode D7 is coupled to thediode D8 and commonly connected at the potential neutral point P10. Thepower switches S25-S28 may be MOSFETs, IGBTs with anti-parallel diodes,or other semiconductor switches. Moreover, the power switches S25-S28and the diodes D7, D8 can be integrated into a modular structure. Inaddition, the third three-level bridge arm 141 and the fourththree-level bridge arm 142 of the DC-to-AC conversion unit 14 can bethree-level bridge arms, thereby reducing switch stress.

The fifth three-level bridge arm 151 has two in-series power switchesS31, S32 and a diode D9 is coupled to a common contact between the powerswitches S31, S32, and two in-series power switches S33, S34 and a diodeD10 is coupled to a common contact between the power switches S33, S34.The power switch S32 is coupled to the power switch S33 and commonlyconnected at a fifth contact P21, and the diode D9 is coupled to thediode D10 and commonly connected at a potential neutral point P20. Thepower switches S31-S34 may be MOSFETs, IGBTs with anti-parallel diodes,or other semiconductor switches. Moreover, the power switches S31-S34and the diodes D9, D10 can be integrated into a modular structure,thereby reducing pins, making the layout easy, and reducing the varianceof the components.

The sixth three-level bridge arm 152 has two in-series power switchesS35, S36 and a diode D11 is coupled to a common contact between thepower switches S35, S36, and two in-series power switches S37, S38 and adiode D12 is coupled to a common contact between the power switches S37,S38. The power switch S36 is coupled to the power switch S37 andcommonly connected at a sixth contact P22, and the diode D11 is coupledto the diode D12 and commonly connected at the potential neutral pointP20. The power switches S35-S38 may be MOSFETs, IGBTs with anti-paralleldiodes, or other semiconductor switches. Moreover, the power switchesS35-S38 and the diodes D11, D12 can be integrated into a modularstructure, thereby reducing pins, making the layout easy, and reducingthe variance of the components. In addition, the fifth contact P21 ofthe fifth three-level bridge arm 151 of the second AC-to-DC conversionunit 15 and the sixth contact P22 of the sixth three-level bridge arm152 of the second AC-to-DC conversion unit 15 are coupled to thesecondary side 132 of the isolated transformer 13 to receive the ACsignal provided from the secondary side 132. Since the operation of thesecond AC-to-DC conversion unit 15 is similar to that of the firstAC-to-DC conversion unit 11, the detail description is omitted here forconciseness.

In one embodiment, the first three-level bridge arm 111 to the sixththree-level bridge arm 152 can be the same modular structure, andtherefore those modular bridge arms can be interchangeably used in thesystem to simplify installation time, prevent assembly errors, andsimplify the design and control strategy of the control circuit.

The primary side 131 of the isolated transformer 13 has an LLC resonanttank. As shown in FIGS. 1A and 1B, the LLC resonant tank is presented insymmetrical parameters, i.e., each branch has a resonant capacitance(2Cr) and a leakage inductance (½Llk1). Alternatively, one branch havinga resonant capacitance (Cr) and a leakage inductance (Llk1) can also bepresented in asymmetrical parameters. The isolated transformer 13 isused as an electrical isolation between the circuits in the primary side131 and the circuits in the secondary side 132.

The power apparatus applied in the SST structure of the presentdisclosure, the first DC voltage Vdc1 of the first DC bus 12 issubstantially equal to or substantially similar to the second DC voltageVdc2, for example but not limited to that the first DC voltage Vdc1 is1,580 volts and the second DC voltage Vdc2 is 1,500 volts. Such avoltage is close to the power generation system of the photovoltaicpower station, and it is suitable for connecting a DC voltage bus of thephotovoltaic power station to perform power conversion and regulation.Accordingly, since the first three-level bridge arm 111 to the sixththree-level bridge arm 152 are the same modular structure (the samespecification), the three-level bridge arms can be replaced by eachother for the use of the system, thereby simplifying installationprocess and simplifying the design and control strategy of the controlcircuit.

The power apparatus has operation modes of bidirectional power flow, andthe bidirectional operation modes include an energy-storing mode (alsoreferred to as forward operation) and an energy-releasing mode (alsoreferred to as reverse operation). The forward operation means that thepower apparatus receives the AC power source Vac (or power provided by agrid), and the AC power source Vac is converted by the first AC-to-DCconversion unit 11, the DC-to-AC conversion unit 14, and the secondAC-to-DC conversion unit 15 into the DC power source Vdc for supplyingpower to a DC load, such as a charging station or an energy storagesystem (ESS). The specific application may be, for example but notlimited to, the electric energy provided from the grid is supplied tothe charging station for charging the electric vehicle, or the off-peakoperation of the grid or the excess power of the decentralized generatorapparatus can be stored in the energy storage system.

On the contrary, the reverse operation means that the DC power sourceVdc is converted by the second AC-to-DC conversion unit 15, the DC-to-ACconversion unit 14, and the first AC-to-DC conversion unit 11 into theAC power source Vac. The specific application may be, for example butnot limited to, the DC power source outputted from the photovoltaic cellis used as compensation for regional peak power demand, adjustment ofpower supply quality, and is even sold to the power grid (electric powercompany). In addition, the circuit of the power supply apparatusexhibits a symmetrical configuration, which simplifies the design andcontrol strategy of the control circuit in the forward and reverseoperations.

In one embodiment, a switching frequency of the power switches S11-S18of the first three-level bridge arm 111 and the second three-levelbridge arm 112 is a first switching frequency, and a switching frequencyof the power switches of the third three-level bridge arm 141, thefourth three-level bridge arm 142, the fifth three-level bridge arm 151,and the sixth three-level bridge arm 152 is a second switchingfrequency. In particular, the first switching frequency is less than thesecond switching frequency. For example, the first switching frequencyis between 7 kHz to 12 kHz, and the second switching frequency isbetween 200 kHz to 400 kHz. Specifically, since the DC-to-AC conversionunit 14, the second AC-to-DC conversion unit 15, and the LLC resonanttank form a resonant conversion circuit, the power switches of the thirdthree-level bridge arm 141 and the fourth three-level bridge arm 142 andthe power switches of the fifth three-level bridge arm 151 and the sixththree-level bridge arm 152 operate in soft switching. Therefore, thesecond switching frequency can be up to between 200 kHz to 400 kHz toreduce the size of the transformer, that is, compared with theline-frequency transformer in the traditional power system, the isolatedtransformer 13 achieves the purpose of electrical isolation and issignificantly reduced in size. In addition, since the power switches ofthe first three-level bridge arm 111 and the second three-level bridgearm 112 of the first AC-to-DC conversion unit 11 operate in hardswitching, the first switching frequency is smaller than the secondswitching frequency, for example between 7 kHz and 12 kHz so that theswitching loss of the first AC-to-DC conversion unit 11 can be reducedto improve the efficiency.

The power apparatus of the present disclosure further includes aseries-connected switch unit 17, and the switch unit 17 is coupled on abranch of a second side of the second AC-to-DC conversion unit 15. Whena plurality of power apparatuses is used in parallel, the switch unit 17can be turned off so that the output of the power apparatuses does notaffect the system voltage of the parallel-connected architecture.

Please refer to FIG. 2, which shows a block diagram of a three-phasepower source system applied in SST structure according to the presentdisclosure. FIG. 2 shows that the three-phase power source systemincludes a plurality of power apparatuses. In the AC power source side,the power apparatuses are connected in series, and the second DC buses16 are connected in parallel. Specifically, take the A phase of thethree-phase power source for example, the number of the powerapparatuses with multi isolated DC power sources is determined by aratio of a system voltage to a withstand voltage of each powerapparatus. For example, if a line voltage of the system voltage is 13.2kV (i.e., a phase voltage is 7.62 kV) and the withstand voltage of eachapparatus is 0.866 kV, the number of the power apparatuses in each phaseis nine. Therefore, the first AC-to-DC conversion units 11 of the ninepower apparatuses are coupled in series and the second DC buses 16 ofthe nine power apparatuses are coupled in parallel to commonly providethe DC power source Vdc (under the forward operation) to supply power tothe charging station or the energy storage system, or to commonlyreceive the DC power source Vdc (under the reverse operation) from thephotovoltaic cell. In addition, each phase architecture shown in FIG. 2can be combined into a three-phase multi-group architecture.Specifically, the AC power source side is connected by a wye (Y)connection and a grounded neutral point N, and each group of thethree-phase power apparatuses can be coupled in parallel. Take the ninepower apparatuses per phase for example, by combining the powerapparatuses in the three phases, the twenty-seven second DC buses 16 areconnected in parallel, thereby achieving the voltage equalization andpower balance. Take the charging station for example, the required powerof the charging station can be supplied by the DC power source Vdcprovided by the twenty-seven power apparatuses. In particular, thetwenty-seven power apparatuses can, for example but not limited to,averagely or proportionally provide the required power for the chargingstation.

Please refer to FIG. 3, which shows a schematic view of an interleavedphase-shift control of a first AC-to-DC conversion unit according to thepresent disclosure. The first AC-to-DC conversion units 11 of each phaseof the three-phase power source system are controlled by an interleavedphase-shift manner. Take three power apparatuses of each phase as anexample, the voltage between the first contact P11 and the secondcontact P12 of each first AC-to-DC conversion unit 11 is controlled as afirst voltage V1, a second voltage V2, and a third voltage V3,respectively. In particular, each of the first voltage V1, the secondvoltage V2, and the third voltage V3 has a plurality of voltage levelsinvolving Vdc1, ½*Vdc1, 0, −½*Vdc1, −Vdc1. Each of the first AC-to-DCconversion unit 11 corresponding to the voltage V1˜V3 is switching in 10kHz with 120 degrees phase-shifted, and therefore a frequency (systemfrequency) of a phase voltage VAN of each phase can be multiplied to 30kHz. Accordingly, each group of the first AC-to-DC conversion unit 11can have a lower switching frequency, which can improve the efficiency,and the system has a better total harmonic distortion (THD) so thatsmaller filter components can be used.

In conclusion, the present disclosure has following features andadvantages:

1. The solid state transformer is used to replace the conventionaltransformer, thereby increasing efficiency and reducing the occupiedvolume.

2. The three-level bridge arms (including the first three-level bridgearm to the sixth three-level bridge arm) form modular structures,thereby reducing pins, making the layout easy, and reducing the varianceof the components.

3. The first three-level bridge arm to the sixth three-level bridge armcan be the same modular structure, and therefore those modular bridgearms can be interchangeably used in the system to simplify installationtime, prevent assembly errors, and simplify the design and controlstrategy of the control circuit.

4. The first three-level bridge arm and the second three-level bridgearm (the third three-level bridge arm and the fourth three-level bridgearm, and the fifth three-level bridge arm and the sixth three-levelbridge arm) can be three-level bridge arms, thereby reducing switchstress and reducing harmonics.

5. Since the DC-to-AC conversion unit, the second AC-to-DC conversionunit, and the LLC resonant tank form a resonant conversion circuit, thepower switches of the third three-level bridge arm and the fourththree-level bridge arm and the power switches of the fifth three-levelbridge arm and the sixth three-level bridge arm operate in softswitching, and therefore the second switching frequency can be up tobetween 200 kHz to 400 kHz to significantly reduce the size of thetransformer.

6. By combining the power apparatuses in the three phases to make thesecond DC buses be connected in parallel, thereby achieving the voltageequalization and power balance.

7. By controlling the first AC-to-DC conversion units of each phase ofthe three-phase power source system in the interleaved phase-shiftmanner to make the first AC-to-DC conversion unit have a lower switchingfrequency, which can improve the efficiency, and the system has a bettertotal harmonic distortion (THD) so that smaller filter components can beused.

Although the present disclosure has been described with reference to thepreferred embodiment thereof, it will be understood that the presentdisclosure is not limited to the details thereof. Various substitutionsand modifications have been suggested in the foregoing description, andothers will occur to those of ordinary skill in the art. Therefore, allsuch substitutions and modifications are intended to be embraced withinthe scope of the present disclosure as defined in the appended claims.

What is claimed is:
 1. A power apparatus applied in a solid statetransformer structure, comprising: a first AC-to-DC conversion unithaving a first bridge arm and a second bridge arm coupled to the firstbridge arm, and a first side of the first AC-to-DC conversion unitcoupled to an AC power source, a first DC bus coupled to a second sideof the first AC-to-DC conversion unit, and having a first DC voltage, anisolated transformer having a primary side and a secondary side, aDC-to-AC conversion unit having a third bridge arm and a fourth bridgearm coupled to the third bridge arm, and a first side of the DC-to-ACconversion unit coupled to the first DC bus and a second side of theDC-to-AC conversion unit coupled to the primary side, a second AC-to-DCconversion unit having a fifth bridge arm and a sixth bridge arm coupledto the fifth bridge arm, and a first side of the second AC-to-DCconversion unit coupled to the secondary side, and a second DC buscoupled to a second side of the second AC-to-DC conversion unit, andhaving a second DC voltage, wherein a switching frequency of the firstbridge arm and the second bridge arm is a first switching frequency, aswitching frequency of the three bridge arm, the fourth bridge arm, thefifth bridge arm, and the sixth bridge arm is a second switchingfrequency, and the first switching frequency is different from thesecond switching frequency.
 2. The power apparatus applied in the solidstate transformer structure in claim 1, wherein any one of the firstbridge arm to the sixth bridge arm is a modular structure.
 3. The powerapparatus applied in the solid state transformer structure in claim 2,wherein the first bridge arm to the sixth bridge arm are the samemodular structure.
 4. The power apparatus applied in the solid statetransformer structure in claim 3, wherein each of the first bridge armto the sixth bridge arm is a three-level bridge arm.
 5. The powerapparatus applied in the solid state transformer structure in claim 2,wherein each of the first bridge arm to the sixth bridge arm is athree-level bridge arm.
 6. The power apparatus applied in the solidstate transformer structure in claim 1, wherein each of the first bridgearm to the sixth bridge arm is a three-level bridge arm.
 7. The powerapparatus applied in the solid state transformer structure in claim 1,wherein the first bridge arm has a first contact coupled to the AC powersource, the second bridge arm has a second contact coupled to the ACpower source, and there is a plurality of different voltage levelsbetween the first contact and the second contact.
 8. The power apparatusapplied in the solid state transformer structure in claim 1, wherein theprimary side of the isolated transformer has an LLC resonant tank. 9.The power apparatus applied in the solid state transformer structure inclaim 1, wherein the first DC voltage is substantially equal to orsubstantially similar to the second DC voltage.
 10. The power apparatusapplied in the solid state transformer structure in claim 1, wherein thepower apparatus has an operation mode of bidirectional power flow. 11.The power apparatus applied in the solid state transformer structure inclaim 1, wherein the first switching frequency is less than the secondswitching frequency.
 12. The power apparatus applied in the solid statetransformer structure in claim 11, wherein the first switching frequencyis between 7 kHz and 12 kHz.
 13. The power apparatus applied in thesolid state transformer structure in claim 11, wherein the secondswitching frequency is between 200 kHz and 400 kHz.
 14. The powerapparatus applied in the solid state transformer structure in claim 1,wherein the second side of the second AC-to-DC conversion unit has aseries-connected switch unit.
 15. A three-phase power system applied ina solid state transformer structure, any one phase of an AC power sourceis connected to a plurality of power apparatuses, and each of the powerapparatuses comprising: a first AC-to-DC conversion unit having a firstbridge arm and a second bridge arm coupled to the first bridge arm, anda first side of the first AC-to-DC conversion unit coupled to an ACpower source, a first DC bus coupled to a second side of the firstAC-to-DC conversion unit, and having a first DC voltage, an isolatedtransformer having a primary side and a secondary side, a DC-to-ACconversion unit having a third bridge arm and a fourth bridge armcoupled to the third bridge arm, and a first side of the DC-to-ACconversion unit coupled to the first DC bus and a second side of theDC-to-AC conversion unit coupled to the primary side, a second AC-to-DCconversion unit having a fifth bridge arm and a sixth bridge arm coupledto the fifth bridge arm, and a first side of the second AC-to-DCconversion unit coupled to the secondary side, and a second DC buscoupled to a second side of the second AC-to-DC conversion unit, andhaving a second DC voltage, wherein a switching frequency of the firstbridge arm and the second bridge arm is a first switching frequency, aswitching frequency of the three bridge arm, the fourth bridge arm, thefifth bridge arm, and the sixth bridge arm is a second switchingfrequency, and the first switching frequency is different from thesecond switching frequency, wherein the first AC-to-DC conversion unitsof the power apparatuses are coupled in series, and the second DC busesof the power apparatuses are coupled in parallel.
 16. The three-phasepower system applied in the solid state transformer structure in claim15, wherein each of the first bridge arm to the sixth bridge arm is athree-level bridge arm.
 17. The three-phase power system applied in thesolid state transformer structure in claim 15, the first AC-to-DCconversion units in each phase are controlled by an interleavedphase-shift manner.
 18. The three-phase power system applied in thesolid state transformer structure in claim 15, wherein the number of thepower apparatuses in each phase is determined by a ratio of a systemvoltage to a withstand voltage of each power apparatus.
 19. Thethree-phase power system applied in the solid state transformerstructure in claim 15, wherein the second DC bus in each phase iscoupled to one of a charging station, a photovoltaic cell, and an energystorage system.